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    <title>四则运算</title>
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    <h1>四则运算</h1>
    数值<input type="text" name=" " id=" " placeholder="请输入第一个数"><br><br>
    数值<input type="text" name=" " id=" " placeholder="请输入第二个数"><br><br>
    结果<input type="text" name=" " id=" " ><br><br>
    <input type="button" value="加" onclick="jia()">
    <input type="button" value="减" onclick="jian()">
    <input type="button" value="乘" onclick="cheng()">
    <input type="button" value="除" onclick="chu()">
    <input type="button" value="重置" onclick="reset()">
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